The present invention relates to a toner for developing electrostatic latent images, in particular a toner prepared by granulating particles in wet process, aggregating and pulverizing the particles.
Copy images of high quality have been required in the field of a copy machine or printer for electrophotography. In order to meet the requirements, it has been researched and investigated to make toner particles small.
A pulverizing method has been conventionally used as a production method of toner. The pulverizing method is carried out by melting and kneading binder resins and coloring materials and then pulverizing and classifying the kneaded materials.
Toner particles obtained by the pulverizing method, however, have broad distribution of particles size of toner. The limitations from technical view points and productivity such as yield are imposed on the pulverizing method.
Wet granulating methods such as a suspension polymerization method and a suspension granulating method are known as a method which can be applied to make toner particles small and more useful than the pulverizing method from the viewpoint of productivity. By the suspension polymerization method, polymerizable monomers, coloring materials and other additives are added to be granulated to give toner particles synthesized in suspension. Such wet granulating methods can give spherical toner particles having small particles and small distribution of particle size. But as the shape of toner is almost spherical, there arise problems such as poor cleaning properties by a blade.
Further there is a difficulty in controlling chargeability, the reason of which is not clearly understood but thought to be attributed to small surface area effective in tribo-charging because of spherical shape and contaminants remaining as impurities such as a surfactant for polymerization and granulation and a catalyst for polymerization.